Adhesive compositions, particularly conductive adhesives, are used for a variety of purposes in the fabrication and assembly of semiconductor packages and microelectronic devices. The more prominent uses include bonding of electronic elements such as integrated circuit chips to lead frames or other substrates, and bonding of circuit packages or assemblies to printed wire boards. Adhesives useful for electronic packaging applications typically exhibit properties such as good mechanical strength, curing properties that do not affect the component or the carrier, and thixotropic properties compatible with application to microelectronic and semiconductor components.
A few methods have been described in the art to decrease the brittleness of bismaleimide thermosets. The well-known Michael addition reaction, using aromatic or aliphatic diamines is one of these methods. Michael addition has been used to reduce the cross-link density and therefore to improve the toughness of these thermosets. The Michael addition approach to toughen BMI resins, however, has at least one significant limitation. Michael addition is thermally reversible and the amine extended bismaleimides will revert to the starting materials in the vicinity of 300° C. This thermal reversion or retro-Michael addition is unacceptable for any adhesive end-use that requires thermal resistance.
The bismaleimides represent one useful class of thermoset compounds that have been used in the microelectronic packaging industry. Bismaleimides are curable, meaning that they are capable of polymerization to yield cross-linked resins. In addition, bismaleimides may be homocured in the presence of free radicals or photoinitiators, or combined with other free-radical curing monomers (e.g., acrylates, methacrylates, syrenics, vinyl ethers, vinyl esters, allyl monomers, olefins, and the like). They may also be cured in the presence of comonomers via, Diels-Alder, -ene, and Michael addition mechanisms.
Commercially available bismaleimide thermoset compositions are noted for their high modulus, and excellent resistance to thermal degradation. However, these thermoset compositions are also well known for brittleness. The utility of the bismaleimide class of thermosets could be vastly improved if less brittle formulations could be achieved that retain the desirable thermal and elastic properties.
A few methods have been described in the art to decrease the brittleness of bismaleimide thermosets. The well-known Michael addition reaction, using aromatic or aliphatic diamines is one of these methods. Michael addition has been used to reduce the cross-link density and therefore to improve the toughness of these thermosets. The Michael addition approach to toughen BMI resins, however, has at least one significant limitation. Michael addition is thermally reversible and the amine extended bismaleimides will revert to the starting materials in the vicinity of 300° C. This thermal reversion or retro-Michael addition is unacceptable for any adhesive end-use that requires thermal resistance.
The imide-extended polymaleimides of this invention are contemplated for use in a wide variety of applications. They can be used, for example, as matrix resins and adhesives for aerospace, marine, automotive, wind turbine, and sports equipment composite products. They can be used in the fabrication of printed wiring boards and flexible circuits. The compounds of this invention can be used in die attach adhesives, underfill and mold compound resins for electronic packaging. They can be used to make thermally resistant films and film adhesives. They may also be used in the fabrication of anisotropic conductive adhesive films and pastes.